Thermal regenerative oxidizers (RTOs) are used in a number of industries to reduce the quantity of contaminants in process effluent gases. RTOs are unique in their ability to conserve fuel through the use of heat exchangers. In an RTO, the process effluent gases are oxidized in a combustion chamber. As the high-temperature combustion gases move to an exhaust stack, they flow through a heat exchanger, typically a chamber filled with ceramic saddles or the like. In the heat exchanger, up to 95% of the heat is transferred from the gases to the ceramic saddles. The flow of gases is then reversed such that the inlet process gases move through the heat exchanger toward the combustion chamber. Heat is transferred from the hot ceramic media to the process gases and consequently less energy is required to oxidize the process gases in the combustion chamber.
Several configurations of RTOs have been developed based on this heat recovery principle. In an RTO having three or more chambers, one heat exchanger sequentially serves as a standby chamber such that the continuous flow of process gas is not interrupted during flow reversal. In a two chamber RTO, however, neither of the heat exchangers can function as a standby chamber and thus the problem of handling a continuous process gas stream is more difficult. In a two chamber RTO both heat exchangers are separately attached to a shared combustion chamber. A flow path is thereby established that extends from the inlet of one heat exchanger, through the heat exchange medium, into the combustion chamber and then out via the second heat exchange chamber. In order for the incoming process gas to capture heat from the heat exchangers, gas flow through the RTO must be periodically reversed. And, as will be appreciated by those skilled in the art, flow reversal must occur in a manner which minimizes discharge of unoxidized process gas to the atmosphere.
The prior art has used electronic and hydraulic controls to actuate valves in RTOs. It is difficult, however, to properly time the opening and closing of the valves associated with the heat exchange chambers and still maintain steady inlet pressures.
Further, hydraulically opened and closed valves tend to significantly restrict the flow of gas through the valves when they first begin to close, but then slowly taper to zero. Accordingly, the valves are restricted in a manner which results in low flow percentages for a relatively long portion of the cycle.
Various types of cams and other mechanical actuation systems have also been used to open and close inlet and outlet butterfly/wafer valves in three chamber RTOs. These have included mechanically operated means which have utilized eccentrically mounted secondary shafts driven by a main shaft.
In the case of two chamber RTOs the most frequently used valve system employs poppet valves actuated by hydraulic or air linear actuators connected to the valve shaft. Poppet valves go from zero flow to full flow quickly and the opening and closing of the poppets minimizes the tendency of foreign particles carried by the gases to be trapped in the valve. Gas moving through the valve is directed by the position of a disc or "poppet" which is fixed on a stem. The disc is moved linearly so that it seats on one of two opposed valve seats.
In two chamber RTOs, two poppet valves are employed, each having its own hydraulic or air linear actuator. It will be appreciated that for efficient operation, both poppet valves must be timed so that they open and close as fast as possible, forming substantially air-tight seals. While hydraulic or air linear actuated poppet valves have some advantages (i.e., the overall simplicity of poppet valves), for large RTOs such systems are not always reliable. For example, in a large RTO a poppet disc may weigh in excess of 300 pounds and may cycle 200,000 times per year. With discs of this size, poppet valves actuated hydraulically or by air linear means are inadequate to provide control and sealing force to the degree required for reliable operation. Moreover, due to the force with which the valves are closed, they may cause premature wear of valve seats, i.e. due to the "slamming" of the disc against the valve seat. Moreover, the lack of constant air pressure in RTOs, the temperature variability of many hydraulic fluids, as speed varies season to season due to ambient variances and occasional frozen air lines, and a number of other factors make these conventional systems less than optimum.
Therefore it would be desirable to provide a two chamber RTO valve system which addresses the problems inherent in the prior art. The present invention meets these objectives.